Field
The disclosed concept pertains generally to converting energy from photovoltaic (PV) arrays and, more particularly, to power conversion systems for a PV system. The invention further pertains to methods of power conversion for a PV system. The invention also pertains to PV systems.
Background Information
Photovoltaic (PV) arrays are typically configured in a series/parallel arrangement of a plurality of PV modules. The conventional practice is to ensure that the generated direct current (DC) voltage of a string of PV modules, under worst case conditions, does not exceed the insulation ratings of the PV modules. For example, the National Electric Code (NEC) requires this voltage to be under 600 VDC.
An example PV array/inverter system 2 is shown in FIG. 1. Each of two example PV arrays 4,6 consists of a number of strings 8 of PV modules 10 electrically connected in parallel before the corresponding PV array is electrically connected to an inverter 12.
The power output of a PV module for a level of solar radiation depends, for example, on the temperature of the PV module, the condition of the PV module surface, the age of the PV module, and the technology of the PV module. However, the general characteristics of the PV array DC voltage (volts) 14, PV array DC current (amperes) 16 and PV array DC power output (watts) 18 with respect to 100% solar radiation (i.e., “insolation”) 20 are shown in FIG. 2. A plot 22 of the array output current versus the array output voltage is also shown.
Referring again to FIG. 1, the DC power from the PV arrays 4,6 is converted into alternating current (AC) power by a power converter (e.g., the inverter 12) before, for example, being injected into a utility grid 24. The power converter 12 preferably ensures that the DC power from the PV arrays 4,6 is maximized. The maximization of energy from the PV arrays 4,6 is done by continuously changing the operating point based on the Sun's radiation and the temperature of the PV modules 10. However, there are various efficiencies of conversion from DC power to AC power. For example, the power conversion can be made with more than one power converter (not shown). When more than one power converter is used, the efficiency is dependent on the operating point of the various power converters. In addition, a sub-array connected to each power converter could have a different operating sub-array voltage for maximum power.
In this power conversion process, there is an additional inefficiency in the transformer 26 between the inverter 12 and the utility grid 24. Example efficiency curves 28,30 of the inverter 12 and the transformer 26 have convex characteristics as shown in FIG. 3. The inverter efficiency depends mainly on the input DC voltage, inverter switching frequency and operating current. The efficiency of the transformer 26 is dependent on the design and operating point.
There is room for improvement in power conversion systems for a PV system.
There is also room for improvement in methods of power conversion for a PV system.
There is further room for improvement in PV systems.